Air-conditioning systems in aircraft are usually supplied with bleed air in the form of hot compressed air that is withdrawn from a compressor stage of an engine or obtained with the aid of a compressor that is driven by an auxiliary gas turbine (“APU”). Such an air-conditioning system is known, for example, from DE 199 36 641 A1.
Such conventional air-conditioning systems are systems that operate at a high temperature and pressure level. The withdrawn bleed air has a temperature level of 200° C. and is distributed to all bleed air consumers in the aircraft via corresponding ducts. Contemporary air-conditioning systems require ram air (“ram air”) as a heat sink. During the operation on the ground, a fan needs to take in air in order to cool the system. The bleed air used for the air-conditioning is cooled by means of this ram air, as well as compression, intermediate cooling and expansion of the bleed air. The air-conditioning units are usually accommodated in an unpressurized bay in the belly of the aircraft. Consequently, the duct system for the air supply in the aircraft may, if so required, have to be installed over great distances. It needs to be ensured over the entire length of the duct system that the ducts conveying the hot bleed air do not negatively affect the materials used.
Since large air-air heat exchangers are nowadays required near the bleed air circuit of the system, the air-conditioning units are usually arranged in the unpressurized bay in the belly of the aircraft such that the thermal energy can be emitted into the surroundings. However, such a heat emission is not advantageous due to the separation into pressurized and unpressurized zones and the associated large bulges in the pressurized aircraft fuselage. Since the air-conditioning system is supplied with hot bleed air, it furthermore needs to be covered with a heat shield in order to isolate and separate these hot components. If the air-conditioning unit is arranged in a zone that contains kerosene vapor, sufficient ventilation of the unpressurized bay (“UBV—Unpressurized Bay Ventilation”) furthermore needs to be ensured.
Cabin zones are supplied with air on a temperature level that is defined by the cabin zone with the lowest required temperature level. Since this air is excessively cold for the remaining cabin zones, hot trim air (“trim air”) is supplied to each cabin and cockpit zone in order to reach the nominal temperature in the cabin zones. In addition to the disadvantageous energy balance, it also needs to be taken into account that the ducts for the hot trim air need to be carefully integrated into the pressurized zone in order to prevent negative effects on less temperature-resistant materials that are nowadays used more and more frequently in aircraft such as, for example, plastics reinforced with carbon fibers.